Displaying drought severity, date, and duration.
This image shows how an extreme drought developed at Manilla, NSW in 2018. The contours and colours indicate drought severity. They show whether the rainfall shortage was extreme, severe, or merely serious. At times without drought, they show when the rainfall was above normal.
[See Note below: “Classes of rainfall shortage”.]
Calendar months appear in order along the top, showing how the severity of drought has changed as time has passed.
Down the side, the duration of drought is shown, from one month duration at the top to thirty-six months duration at the bottom. Each month is shown on one vertical line, with a severity value plotted against each duration value.
[In other posts (like “Record 15-month Drought in 2018”) each individual month’s drought data is plotted as severity versus duration.]
The pattern of the 2018 drought
In each of the months May, June, and July 2018, the monthly rainfall was a serious or severe shortage, below the 10th percentile. As rainfall had been below normal also in March and April, each month after April saw severe and even extreme shortages that extended to durations of 3 months and longer. By August, there were extreme droughts of five-month and six-month duration, despite reasonable rain (28 mm) having fallen in August.
By September, the nine months to date were in extreme drought. That is to say, the first nine months of 2018 had a total rainfall lower than any but 1% of all nine-month periods in history.
However, in that month (September 2018), the rainfall shortage for a 12-month duration was not extreme, but merely severe. Yet the 15-month shortage for that month was also extreme. In fact, it was the lowest 15-month rainfall total in history (400 mm).
The source of the extreme 15-month shortage at this date is obvious. One year earlier, the month of September 2017 had a serious rainfall shortage, and the two previous months had little rain. From these short-term shortages in spring 2017, shortages of longer duration descend as an arc across the graph. They become mild nine-month shortages during the summer, then worsen to extreme by merging with 2018 shortages.
Manilla’s rainfall is seasonal, with two distinct modes, each dominating half the year. There is a major summer (monsoonal) mode and a minor winter (westerly) mode. The summer rainfall mode dominates from October to March and the winter rainfall mode dominates from April to September. [See Note below: “Manilla’s rainfall seasonality”.]
This graph documents a failure of winter rainfall. The 2018 months that had rainfall shortages were the months of dominance of the winter rainfall mode (April to September). Rainfall had been near or above normal in the preceding summer rainfall mode (October 2017 to March 2018), and was so again in October 2018. [See Note below: “Limitations of this analysis”]
The rainfall shortages in the previous year (2017) were also restricted to the months of the winter rainfall mode, but began very late. Shortages did not develop until the final three months (July, August, September). Only when the extreme drought of 2018 developed did those 2017 shortages have a big effect.